Hole production is a common step in semiconductor device manufacturing. The holes are typically used to make electrical connections to a semiconductor or metal layer through an overlying non-conducting (dielectric) layer, such as an oxide layer. In order to produce holes, a layer of photoresist is first deposited on the wafer surface. The photoresist is exposed to patterned visible or ultraviolet radiation, hardened, and developed in order to form a “mask” over the wafer, with mask patterns corresponding to hole locations. Then the wafer is transferred to an etch station where holes are formed through the dielectric layer, down to the underlying semiconductor or metallic layer. The photoresist mask is then removed, and the holes are filled with metal. A similar masking and etching process is used in producing trenches or vias in the wafer surface.
In order to ensure consistent device performance, the depth, width, and bottom surface of contact openings must be carefully controlled at various locations across the wafer surface. (In the context of the present patent application and in the claims, the term “holes” refers to all structures of the type described above, including contact openings, vias, and trenches.) Deviations in the dimensions of contact openings at a location on the wafer or across the wafer surface can lead to variations in the contact resistance. If these variations are too large, they impact on device performance and can lead to loss of process yield. The manufacturing process must therefore be carefully monitored and controlled, not only in order to detect deviations in formation of contact openings on individual devices, but also to monitor non-uniformities across the wafer surface. Early detection of process non-uniformity allows the device manufacturer to take corrective action, so as to ensure uniformly high yields and avoid the loss of costly wafers in process.
It is known in the art to use a scanning electron microscope (SEM) to inspect holes formed in a wafer. As holes are typically much deeper than they are wide, a special high aspect ratio (HAR) imaging mode is used. Usually, HAR imaging is used to measure or review hole dimensions such as top and bottom width and wall thickness. However, HAR imaging has some limitations. One limitation is that HAR imaging quality of the bottom of holes is significantly limited when the hole bottom is not connected to ground. Moreover, HAR imaging does not yield quantitative information regarding the degree of hole bottom blockage by different types of residue, such as dielectric materials, photoresist materials, etc. Consequently, the ability to control the etching process based on HAR imaging is very limited and is not quantitative.
An alternative method for hole inspection is described by Yamada et al., in “An In-Line Process Monitoring Method Using Electron Beam Induced Substrate Current,” in Microelectronics-Reliability 41:3 (March 2001), pages 455-459, which is incorporated herein by reference. The compensation current in an electron beam system, also known as the specimen current, is defined as the absorbed current that flows from the primary electron beam to earth via the specimen (i.e., via the wafer). In other words, the specimen current is equal to the difference between the primary beam current and the total electron yield of the specimen due to secondary and backscattered electrons. The specimen current can be either positive or negative, depending on whether the energy of the primary electron beam is in the positive- or negative-charging domain of the specimen. Yamada et al. directed an electron beam at single holes and groups of holes in a silicon oxide surface layer overlying a silicon substrate, and measured the resultant compensation current. They found that the compensation current was a good indicator of hole-bottom oxide thickness, as well as of the hole diameter.
Yamada et al. describe further aspects of hole measurement in U.S. Patent Application Publication No. US 2002/0070738 A1, whose disclosure is incorporated herein by reference. Semiconductor devices are inspected by measuring the specimen current in an area of a sample having no holes as a background value, and comparing this value to the current measured in the area of a hole. The current waveform is automatically evaluated in order to determine whether the measurement is indicative of a defect of the device or of manufacturing equipment used in producing the device.
A disadvantage of some prior art current specimen based methods for assessing holes is their relatively modest throughput. In addition, the generation of very small spots is costly and requires complex tools.